1. Field of the Invention
The present invention relates to a frequency modulation circuit and a semiconductor device including the frequency modulation circuit.
2. Description of the Related Art
In a wireless communication system, there is performed a frequency modulation such as FSK (Frequency Shift Keying) for performing transmission of binary coded information. For example, a PLL frequency synthesizer having a PLL (Phase Locked Loop) circuit is used as a modulation circuit performing the frequency modulation. The PLL frequency synthesizer may have an occurrence of modulation index fluctuation caused by changes in an environment surrounding the system at such an occasion of frequency modulation of an input data i.e. the binary coded information. Thus, there has been devised a frequency modulation circuit having a function of calibrating (correcting) the modulation index (For example, Japanese patent application kokai No. 10-22736).